Relapse to opioid addiction in patients with HIV contributes to drug-seeking behavior, non-adherence to antiretroviral therapy (ART), treatment failure, and the development of viral resistance. Successful addiction treatment should do the opposite. Opioid addicted patients receiving ART are often maintained on methadone or buprenorphine, but these medications are subject to regulatory limitations, do not always stop opioid use, are not available in many settings, and not acceptable to all patients. Naltrexone maintenance is another option that could be useful as it blocks opioid effects and prevents relapse if taken as directed. It has been available as a 50 mg tablet that is taken daily and was initially met with enthusiasm by clinicians; however enthusiasm waned when most patients stopped taking it and relapsed. Efforts to develop long-acting formulations that might overcome these problems were realized only very recently, and sustained release formulations are now available and have renewed interest in using it for opioid addiction treatment. These formulations may also facilitate adherence to ART in opioid addicted patients by preventing relapse. Accordingly, this project aims to test whether an implantable formulation that slowly releases naltrexone and blocks opioid effects for 3 months (IN) is more effective than 50 mg/day oral naltrexone (ON) for improving ART adherence and treatment outcome in opioid addicted patients who are beginning their first episode of ART. Russia is an ideal place to study IN since it has an approved product, most HIV patients are heroin addicts, Russian law does not permit agonist treatment, patient interest in naltrexone is high, and the collaborating research team at Pavlov Medical University has extensive experience with IN and ON. To test the hypothesis that IN results in better HIV treatment outcome than ON, we propose a 48-week, randomized, double blind, double- dummy trial comparing IN with ON in 200 recently detoxified heroin addicts beginning their first episode of ART in St. Petersburg. The primary outcomes will be the proportion of patients with a viral load of <400 copies at 24 and 48 weeks. We hypothesize that IN will work better than ON because it will be associated with less relapse and more adherence.